Smoke detectors are generally used for the early detection of fires. Optical fire detectors are among the most frequently used detectors in the field of fire detection. They can be designed as transmitted-light detectors or as scattered-light detectors. Smoke detectors based on the scattered-radiation principle detect smoke particles by measuring radiation scattered on these smoke particles. The response characteristic, i.e. the sensitivity of all optical smoke detectors, is strongly dependent on the type of fire. The amount, the nature and the composition of the smoke produced by the fire play a large role for the sensitivity of the smoke detector. Fires with low smoke production cannot be detected as well as fires in which a great deal of smoke is produced. In addition, scattered-light smoke detectors have to rely on the circumstance that light will be reflected on the smoke particles. To achieve a more uniform response characteristic of fire detectors, optical smoke detectors can be combined with detectors based on other principles. For example, ionization smoke detectors or temperature detectors are known. These different types of fire detectors can be mounted at different locations in an area, or can even be integrated in a single detector.
Such combinations of optical smoke detectors with temperature detectors or ionization smoke detectors are known. In addition to an increase in temperature and the development of smoke, the appearance of gaseous combustion products is a further significant feature for fire detection. These combustion products can be detected by various types of gas sensors. An object of the present invention is to provide a fire detector which can reliably detect various types of fires, with and without smoke production.
The fire detector of the present invention offers the advantage that the combination of two different sensor methods permits more reliable fire detection than is the case with conventional smoke or fire detectors. Thus, a generally known scattered-light receiver for detecting smoke is combined with at least one further optical receiver which, due to the interposition of a gas-sensitive layer, reacts to specific constituents in the air which typically develop during the combustion. By using a shared light source as optical transmitter, the fire detector can have a very compact and space-saving design. The signal processing of a downstream evaluation unit is also simplified. Furthermore, it is generally sufficient to provide only one such fire detector per area, if the area does not exceed a certain size, instead of several smoke detectors operating on different measuring principles, which considerably simplifies installation and cabling. Additionally, the optical receivers located in the direct radiation range of the optical transmitter can act as transmitted-light smoke detectors, and are thus able to register brightness variations because of aerosols present in the air. This is advantageously permitted by an evaluation unit which is connected downstream of the optical receiver and which evaluates fluctuations of the electrical signal because of fluctuations in the brightness of the received light signal. In so doing, known methods such as modulated measurement or lock-in technique are used.